DE-A 36 43 751 discloses aqueous epoxy resin dispersions which are prepared in the presence of a condensation product of an aliphatic polyol and an epoxide compound as emulsifier. However, these aqueous dispersions include not only water but always a certain quantity of organic solvents. DE-A 41 28 487 describes a method of reducing further this quantity of organic solvents while at the same time improving the properties of these dispersions. For this purpose, the base resin is no longer condensed in the presence of the emulsifier, and flexibilizing polyepoxides are used as raw materials for the epoxy resin. Polypropylene glycol diglycidyl ether is employed in the examples of DE-A 41 28 487.
These dispersions have a markedly reduced organic solvent content with improved performance properties. In connection with endeavors to minimize further and further all the time the amount of volatile organic components in aqueous coating compositions, however, even the reduced solvent content proves to be problematic. Moreover, in order to flexibilize the base resin it is necessary to employ expensive raw materials, which may in addition have adverse effects on the profile of properties.
Cold-curing systems in particular, such as, for example, those used for corrosion prevention and buildings protection, or those subjected to forced drying, receive a particularly critical inspection from the standpoint of solvent emission, since in this case it is not possible, as with stoving systems, to carry out thermal incineration of the solvent afterwards.
A substantial improvement of the existing systems, therefore, would be a binder based on a completely organic solvent-free dispersion, which can be employed for the sector of cold-curing coatings.
The solvents employed serve primarily, in general, as coalescence agents, which slowly evaporate from the film after it is formed and therefore give rise to unfavorable working conditions from the standpoint of industrial hygiene and to unwanted emission of organic constituents into the atmosphere. In addition, the coating unavoidably retains solvent residues, which adversely affect its properties. Without such solvents, however, film formation and leveling are poor, leading to cloudy, poorly adhering films.
The fact that reactive diluents can have a positive effect (reducing viscosity) in conventional epoxy resin formulations and water-dilutable liquid resins is familiar to the person skilled in the art. However, their use in epoxy solid-resin dispersions has been rarely described. U.S. Pat. Nos. 4,315,044, U.S. Pat. No. 4,399,242 and U.S. Pat. No. 4,608,406 describe the use of C.sub.8 -C.sub.80 -alkyl monoepoxides in nonionically stabilized epoxy solid-resin dispersions for curing at room temperature or with heating. In the abovementioned patents, these reactive diluents are stirred into the finished dispersion for improving the shear stability, the freeze-thaw stability, the storage stability and the gloss of the coatings, although some of these effects can only be achieved satisfactorily by the additional use of glycols and/or glycol ethers.
Not mentioned, in contrast, is the function of the reactive diluents as coalescence agents. In particular, there is no mention of the fact that addition to the resin prior to the actual dispersion process is particularly advantageous.
The subsequent admixture of substances to a finished, nonionically stabilized solid-resin dispersion, especially when it has a high solids content, is a very awkward operation, since it requires the thorough homogenization with one another of components of different viscosities and polarities, something which does not take place satisfactorily in relatively large mixing units. Moreover, it is not possible in this way to achieve so high a degree of mixing of resin and reactive diluent, since these components are present in separate micelles.